An optical disk device for carrying out reproduction and recording of an optical disk includes an optical pickup for irradiating an optical disk with laser light for reproduction/recording, or receiving reflection light from the optical disk. The optical pickup includes a photoreceptor and a photoreceptor amplifier circuit. The photoreceptor converts the received reflection light or monitored laser light emitted from a laser light source into an electric signal, and the photoreceptor amplifier circuit amplifies this converted electric signal. Further, to take its advantage of easily realizing a small-sized optical pickup, a recent optical pickup often uses an IC in which a photoreceptor and a photoreceptor amplifier circuit are integrated in a single chip.
FIG. 8 illustrates a conventional photoreceptor amplifier circuit including a differential amplifier circuit. In this photoreceptor amplifier circuit, first, light received by a photodiode PD21 is converted into a photocurrent, and is further converted into a voltage in an inversion amplifier circuit (current-voltage conversion circuit) in which a differential amplifier circuit is connected to a feedback resistor R22, thereby generating an output voltage Vo proportional to the photocurrent. The differential amplifier circuit in the photoreceptor amplifier circuit includes transistors Q21 through Q25, and constant current generators CS21 and CS22.
The NPN-type transistors Q21 and Q22 are provided as a pair of differential transistors. The PNP-type transistor Q23 and Q24 constitute a current mirror circuit, and supply a collector current to the transistors Q21 and Q22, respectively. An input voltage Vin, as a reference voltage, is supplied to the base (non-inversion input terminal of a differential amplifier circuit) of the transistor Q21 via an input resistor R21. The photocurrent of the photodiode PD21 is supplied from the emitter of the transistor 25 via the resistor R22. Further, the constant current generator CS21 supplies a constant current so that the sum of the emitter currents of the transistors Q21 and Q22 becomes a constant value. The NPN-type transistor Q25 serving as an output transistor forms an emitter follower circuit, and outputs an output voltage Vo from a collector of the transistor Q24. The constant current generator CS22 supplies a constant current to the transistor Q25.
The following explains an offset voltage of this differential amplifier circuit. Note that, in the following explanation, all NPN transistors have a common characteristic, and all transistors have a common characteristic. Thus, variation between the transistors is not taken into account.
In the differential amplifier circuit shown in FIG. 8, the output current Vo is found as follows when no voltage is inputted.Vo=Vin−(Ibn1×Rf)−Vben1+Vben2+(Ibn2×Rf)=Vin −(Rf×Icn1/hFEn)−VT×ln(Icn1/Is)+VT×ln(Icn2/Is)+(Rf×Icn2/hFEn)=Vin+((Icn2−Icn1)×Rf/hFEn)+VT×ln(Icn2/Icn1)  (1)
The parameters in the above figure are detailed below.                hFEn: rate of current amplification of NPN transistor        VT: thermoelectromotive force expressed as kT/q(k=Boltzmann's constant, T=absolute temperature, q=electronic charge)        Ibn1: base current of transistor Q21        Ibn2: base current of transistor Q22        Vben1: voltage between base-emitter of transistor Q21        Vben2: voltage between base-emitter of transistor Q22        Icn1: collector current of transistor Q21        Icn2: collector current of transistor Q22        Icp1: collector current of transistor Q23        Icp2: collector current of transistor Q24        Ibp1: base current of transistor Q23        Ibp2: base current of transistor Q24        Ibp3: base current of transistor Q25        Is: reverse saturation current of transistor (a constant depending on the structure etc. of the transistor)        Rf: resistance values of resistors R21 and R22        
Accordingly, the voltage difference between the output voltage Vo and the reference voltage Vin, that is the offset voltage Voff, is denoted as follows.Voff=Vo−Vin=((Icn2−Icn1)×Rf/hFEn)+VT×ln(Icn2/Icn1)  (2)
This equation reveals that the offset voltage is caused by the difference between Icn1 and Icn2. On the contrary, when Icn1=Icn2, Voff becomes 0 and the offset voltage is not generated.
FIG. 9 illustrates another conventional photoreceptor amplifier circuit.
This photoreceptor amplifier circuit has the same structure as that of the circuit of FIG. 8, but additionally includes a NPN-type transistor Q26, a PNP-type transistor Q27, and constant current generators CS23 and CS24. Further, the transistor Q26 and the constant current generator CS23 are serially connected between the power supply voltage Vcc and the ground line. The transistor Q27 and the constant current generator CS24 are serially connected in the same manner. The base of the transistor Q26 is connected to the collector of the transistor Q23, and the base of the transistor Q27 is connected to the collector of the transistor Q24.
In this circuit shown in FIG. 9, Icn1 becomes identical to Icn2 by compensating the value of Ibp1+Ibp2, that is the base current of active load, so that the sum of those becomes Ibp3 (Ibp3=Ibp1+Ibp2); and also compensating Ibp3, the base current of output circuit, using a correcting current Ibn4 (Ibn4=Ibn3). As a result, Icn1 becomes equal to Icn2, thus obtaining Voff=0. Here, considering the Early effect of the transistors Q21 and Q22 constituting a differential pair, the condition Vben1=Vben2 cannot be obtained only by correction of base currents of the active load and the output circuit even when Icn1=Icn2. More specifically, when the collector—emitter voltages of the transistors Q21 and Q22 are respectively expressed as Vben1 and Vben2, they are found as follows.Vben1=VT×ln(Icn1/(Is×(1+(Vcen1/VA)))Vben2=VT×ln(Icn2/(Is×(1+(Vcen2/VA)))
Accordingly, the offset voltage Voff is found as follows.Voff=Vo−Vin=((Icn2−Icn1)×Rf/hFEn)+VT×ln((Icn2×(1+(Vcen1/VA)))/(Icn1×(1×(Vcen2/VA))))  (3)
When Icn1=Icn2 in Formula (3), the offset voltage Voff is expressed as follows.Voff=VT×ln((VA+Vcen1)/(VA+Vcen2))  (4)
However, in Formula (4), the condition Vcen1=Vcen2 must be met to obtain an offset voltage=0.
Here, the following explains the relation between the Early effect and the offset voltage. First, the following relation is found based on the first line of FIG. (1).Voff=Vo−Vin=((Icn2−Icn1)×Rf/hFEn)+(Vben2−Vben1))  (A)
Meanwhile, Vben differs depending on whether the Early effect of transistor is taken into account. Vben in consideration of the Early effect and that ignoring the Early effect are respectively found as (B) and (C) below. This theory is applied also to Vben2.Vben1=VT×ln(Icn1/Is)  (B)Vben1=VT×ln(Icn1/(Is×(1+(Vcen1/VA)))  (C)
Formula (2) is obtained by applying Formula (B) to Formula (A), and Formula (3) is obtained by applying Formula (C) to Formula (A).
When the Early effect is not taken into account, Voff=0 in the circuit of FIG. 9. On the other hand, when the Early effect is taken into account, offset voltage is generated as shown in Formulas (1) through (3).
Since the photoreceptor amplifier circuit of FIG. 9 cannot satisfy the condition Vcen1=Vcen2, the photoreceptor amplifier circuit of FIG. 10, which satisfies Vcen1=Vcen2, has conventionally been used.
The photoreceptor amplifier circuit of FIG. 10 further includes NPN-type transistors Q28 and Q29 and a bias power source E, in addition to the structure of the photoreceptor amplifier circuit of FIG. 9. The transistor Q28 is connected between the transistors Q21 and Q23, and the transistor Q29 is connected between the transistors Q22 and Q24. The bias power source E is connected between the power source line and the respective bases of the transistors Q28 and Q29, and generates a bias voltage VB.
In this photoreceptor amplifier circuit, because of the transistors Q28 and Q29, the collector voltages VA of the transistors Q21 and Q22 both satisfy VA=Vcc−VB−Vben, thereby obtaining Voff=0.
Note that, Document 1 (FIG. 1) and Document 2 listed below are prior art documents disclosing circuits with a similar function to that of the circuit of FIG. 9, though their structures are not identical to FIG. 9. Document 1 discloses a differential amplifier circuit in which correction of base current and output operation are performed by a single common transistor. Document 2 discloses a circuit in which transistors constituting the current mirror circuit and the output transistor are all NPN-type transistors, and the correction of base current is performed by a single NPN-type transistor.
Further, Documents 3 through 7 listed below are prior art documents disclosing circuits with a similar function to that of the circuit of FIG. 10, though their structures are not identical to FIG. 10. These documents each teaches a similar differential pair to the differential pair constituted of the transistors Q21 and Q22, which is arranged so that the pair of circuits have an equal collector voltage.
Document 1: Japanese Laid-Open Patent Application Tokukai 2000-114888/2000 (published on Apr. 21, 2000)
Document 2: Japanese Laid-Open Patent Application Tokukaihei 08-130421/1996 (published on May 21, 1996)
Document 3: Japanese Laid-Open Patent Application Tokukaihei 05-14075/1993 (published on Jan. 22, 1993)
Document 4: Japanese Laid-Open Patent Application Tokukaihei 08-70221/1996 (published on Mar. 12, 1996)
Document 5: Japanese Laid-Open Patent Application Tokukaihei 04-127703/1992 (published on Apr. 28, 1992) (corresponding U.S. Pat. No. 5,144,169A1)
Document 6: Japanese Laid-Open Patent Application Tokukaihei 04-119005/1992 (published on Apr. 20, 1992)
Document 7: Japanese Laid-Open Patent Application Tokukaihei 04-129306/1992 (published on Apr. 30, 1992)
The front monitor element of an optical pickup is a laser intensity monitoring element which outputs a monitor voltage according to the intensity of received laser beam. The laser driver controls a driving current for the laser light source based on the monitor voltage fed back from the front monitor element so that the laser light source emits a laser beam at a predetermined intensity.
With the higher recording speed in recent years, the laser power for recording is increasing. However, there is a limit for output voltage of the front monitor element; for example, a monitor with a power source voltage=5V and a reference voltage=2.5V outputs a voltage less than 2.5V. Accordingly, when the amount of incident light increases due to an increase of laser power, the value of output voltage does not increase according to the intensity of incident light unless the sensitivity of the front monitor element is decreased. On the other hand, the laser power for reproduction is not required to be increased as much as the recording power, and therefore the decrease in sensitivity of front monitor element in accordance with the recording laser power results in a decrease in output signal for reproduction. Therefore, in order to, particularly upon reproduction, to output a voltage precisely reflecting the laser power, it is necessary to decrease the offset voltage and the offset voltage temperature characteristics.
In view of this, the differential amplifier circuit of FIG. 10 is arranged so that Icn1 is equal to Icn2 by meeting the condition: Ibp3=Ibp1+Ibp2, Ibp4=Ibn3, and also Vcen1 is equal to Vcen2 by addition of the transistors Q28 and Q29, thereby obtaining Voff=0. However, this photoreceptor amplifier circuit stops moving when the input voltage Vin exceeds VA+Vben1, and Vcen1 becomes 0. That is, in this photoreceptor amplifier circuit, the range of input voltage for starting operation is limited because of the presence of the transistors Q28 and Q29, resulting in incapability of processing a large signal.